Grade 11 eost answer scheme

GURU CHARAN KUMAR/IB ESS 2012

End of Semester Test November 2012
GRADE 11 ANSWER SHCEME
Question 1
(a)

interval between 1st and 2nd peak is about 90 days and interval
between 2nd and 3rd peak is approximately 75 days;
90 75
= 82.5 80 (± 4 days)/15 January + 80 days ≈ 20 April (± 5 days)
2
(b)

2

(e)

(i)

25 to 30 September; (±3 days)

1

10 December; (±2 days)

1

(i)

15 days; (±5 days)

1

(ii)

(d)

1

(ii)

(c)

T. occidentalis: 7; (±1)
E. sexmaculatus: 1550; (± 50)
Both needed for [1].

the predator population takes some days to take advantage
of the increase in prey;
availability of food facilitates reproduction of prey;
predator cannot increase in number without access to
food and so must wait for increase in prey;

1 max

the increase in prey leads to a corresponding increase in
predator which corrects the trend towards increase in prey;
the decrease in prey leads to a corresponding decrease in
predator which corrects the trend towards decrease in prey;
the periodic nature of the population curves indicates a
feedback controlled interaction;

2 max

elimination of predators/competitors;
modern medicine decreasing effect of disease;
elimination of density-dependent limiting factors;
importation of food or resources from other areas to
overcome shortages;
increased efficiency in utilization of resources;
our ability to colonize almost any habitat;
tools and technology;

3 max

melting of polar ice caps causes lowering of planetary albedo thus
increasing amount of solar energy at Earth’s surface, leading to
increase in temperature;
melting of tundra through warming causes release of methane
causing more warming;
increased evaporation leading to increased precipitation at poles
triggering net cooling;

2 max

(i)

(ii)

(f)

Page 1 of 7

[14]

Question 2
(a)

(i)

figure 2 – marine food webs because there are more links
in the food web;
there are more tropic levels;
there is more than one food source at each tropic level;
there is no organism dependant on a single food type;
Award [2] for correct food web and two correct reasons,
award [1] for correct food web and one correct reason.
(ii)

(iii)

(b)

3 max

if rabbits were removed grass would increase dramatically;
succession would occur: grass – scrub – trees;
fox population would crash and disappear without a food source;

2 max

if foxes were removed the rabbit population would rise
dramatically;
rabbits would overgraze their food source (grass) and the
rabbit population would crash;
dynamic balance between rabbits and grass may result;

2 max

(i)

Answer will different according to the students

1

(ii)

Answer will different according to the students

1
[9]

Question 3
(ii)
(b)

(c)

an open system exchanges energy and material,
a closed system exchanges energy but not material;

sunlight converted by producers;
producers eaten by consumers;
some energy released as heat;
consumers eaten by other consumers;
detrital decay;
(i)

For example coral reefs:
coral reefs increasingly seen as an economic
resource rather than a subsistence resource;
coral reefs have been used for increasingly
intensive fishing;
source of building materials;
sites for tourism;
pollution sink;
cultural and technological change for example
in fishing industry (dynamite fishing);
political motives behind promoting tourist opportunities;
under increasing population pressure;
Page 2 of 7

1

3 max

has led to overfishing (unsustainable);
loss of biodiversity;
damage to extremely vulnerable polyps;
and knock-on effects throughout the ecosystem;

4 max
[7]

Question 4
(a)

Allow any reasonable method.
description of method for non-motile organism (plants, mussels):
measure area;
use of quadrats;
count organisms within area;
calculate number per unit area;
evaluation:
problems such as organisms part-in/part-out of quadrat;
difficulties of very large organisms (e.g. trees);
problem of organisms where it is impossible to clearly distinguish
individuals (e.g. some plants);
for motile organisms:
measure out area/use naturally limited area such as small pond;
use Lincoln index;
trap organisms and mark;
release and allow to mix with population;
retrap, counting number of marked individuals;
use formula to calculate number
no. marked at t 1 no. captured at t 2
population =
;
no. marked organisms caught at t 2
use aerial photos for large organisms (trees; possibly waterfowl);
evaluation:
e.g. for Lincoln index method: problem of births and deaths
between trappings;
inward or outward migration;
some organisms become habituated to trapping/“trapshy”;

5 max

Generally [3] for description and [2] for evaluation unless
account of method is very detailed; in which case up to [4]
may be awarded for method and [1] for evaluation.
No mark for organism’s name; it is intended simply to focus
attention. However, responses must apply to organism selected
– if not mark out of [3].
(b)

(c)

some indices of diversity/e.g. Simpson’s index;
take account of both the abundance of species in an ecosystem
and the number of different species present;
carefully observe/measure specimen (colour, length, number
of limbs) etc.;
use key/by working through logically comparing characteristics
Page 3 of 7

2

of specimen with those in key;
compare specimen with illustrations in field guide/reference
collection or herbarium;
characteristics such as sound (birds, insects)/distribution/
time of year;
good practice recommends that as far as possible identification
should be done in the field and removing/killing/picking
specimens should be avoided;
any other reasonable answers;
(d)

(i)

name of ecosystem and brief description
Must be reasonably specific: “lake” is insufficient; “Lake
Neerabup, north of Perth, Western Australia” gets the mark.

(ii)

(iii)
(iv)

(v)

(e)

(i)

2 max

description of human activity;
must be appropriate to example: for above, “cultivation of
peat surrounding lake for vegetable-growing”;

1

1 max

abiotic factors: e.g. temperature/turbidity/pH/salinity
Must be appropriate.

1

must establish clear link between activity and factor;
and indicate direction or nature of change (e.g. increase
or decrease);

2

for above lake example: test pH by noting colour
change using pH-testing kit;
perform a number of tests and take mean;
ideally test before and after change and note difference;
and at different sites;
Method must be appropriate to example.
Any reasonable example allowable. No marks if response
gives a biotic factor.

2 max

Trophic levels are the feeding position in a food chain such as primary
producers, herbivore, primary carnivore, etc.
Green plants form the first trophic level, the producers.
Herbivores form the second trophic level, while carnivores form the third and
even the fourth trophic levels.
2 max

(ii)

mark out transect across gradient using tape or similar;
establish quadrats at appropriate distances apart along transect;
apply (Simpson’s) diversity index to each quadrat;
N ( N 1)
D=
;
n( n 1)
where D = diversity index, N = total number of organisms
of all species n = number of individuals of a particular species;
Allow [1] if diversity index is well explained, even if the
concept of environmental gradient is misunderstood

Page 4 of 7

2 max

Responses need not have formula completely accurate for
full marks, providing method is clearly understood.
[20]

Question 5
(a)

open, due to inputs/outputs of matter/air/water (and energy);
(b)

inputs would include more CO2 than outputs;
due to net productivity/photosynthesis; (E)
inputs would include less O2 than outputs;
due to net productivity/photosynthesis; (E)
inputs would include more liquid water than outputs;
due to absorption by plants; (E)
inputs would include less water vapour than outputs;
due to evapotranspiration; (E)
inputs would include more light energy/electricity;
whereas outputs would include more heat energy;
due to respiration; (E)
due to conversion of light/electrical energy to heat; (E)
inputs greater than outputs in early stages;
due to growth occurring/biomass increasing; (E)
Any other appropriate differences
[2 max] for differences
[2 max] for appropriate explanation.

(c)

negative (feedback);

(d)

(i)

(ii)

1

4 max

1

in energy transformations (in food chains), not all energy
can be converted to a useful form/some chemical energy
(in food chains) is lost as heat energy;
so not all energy is transferred from one trophic level to the
next;
so the energy available to each successive trophic level decreases;

2 max

the size of the ecotron limits the size of the primary producer
community;
which limits the amount of energy that can be made available
at the bottom of the food chain;
and since there are losses at each trophic level, this limits the
number of trophic levels that can be supported;

2 max
[10]

Page 5 of 7


Question 6
(a)

the amount/range of living diversity per unit area;
including species, habitat and genetic diversity;

2

(b)

number of individuals of a particular species;

1

(ii)

a stable/mature habitat;

1

(iii)

D would decrease;

1

(iv)

(c)

(i)

recent extinction event;
early colonisation;
monoculture;
extreme environmental conditions;

(i)

(ii)

a representative sample of quadrats would be laid out within
the ecosystem;
all vegetative material would be collected from within the quadrat;
the material would then be weighed;
total biomass can be estimated based on material found
within quadrat samples;
method could be repeated several times during the year
to compensate for variations in growth rate within seasons
(if applicable);
Method should be appropriate to the named ecosystem. If a
named ecosystem is not stated then mark out of [2].
capture/mark/release/recapture (Lincoln Index);
sampling strategies would need to be representative of
variations in habitat;
and seasonal variation in mammal abundance;

1 max

3 max

2 max
[11]

Question 8
(a)

(i)

most fish will be found in places where temperatures are
moderate;
normal distribution of fish;
some fish will be able to survive at more extreme temperatures
(but they will be in a state of physiological stress);
(ii)
(iii)

(b)

2 max

at low and high temperatures, only small populations of
fish are supported due to physiological stress

1

density independent because temperature is a factor which
affects populations regardless of population size

1

Accept any reasonable response. Answers could include:
salinity/water pH/flow velocity/wave action/soil pH/
light intensity/wind speed/soil moisture/drainage/slope/
Page 6 of 7

particle size/mineral content
4 correct = [2], 2 or 3 correct = [1], 1 correct = [0]
(c)

For example, flow velocity:
use a flow meter;
insert into water just below surface and take a number of readings
to ensure accuracy;
results can be misleading if only one part of stream is measured;
water flows can vary over time and with depth;

Page 7 of 7

2 max

3 max

Grade 11 eost answer scheme

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